This invention refers to a single phase asynchronous motor with two magnetic poles provided with a cage rotor and, more specifically, a new construction for the stator in a motor of said type.
Single phase asynchronous motors with two poles provided with a cage rotor have two windings, that is, the main and the auxiliary coils of the stator. The auxiliary coil is designed to overcome the resistant torque while the rotor is started and bring same to a rotation speed close to its synchronous rotation speed. At this time said coil is deenergized, and remains so until it is required to operate again due to an excessive overload or a new motor starting. This low utilization factor of the auxiliary coil makes it desirable that this auxiliary phase occupy a minimum space volume as possible in the motor and the material consumption thereof is as small as possible.
For these reasons, stator plates are die-stamped with slots in different sizes. FIGS. 2 and 3 in the attached drawings show two different configurations of plates usually employed and the die cutting layout thereof. As seen in FIG. 2, the height of the stator plate crown is not necessarily constant, since the magnetic flux density has its maximum value towards the slots in the main coil (PP axis), while the minimum value thereof is towards the slots in the auxiliary coil (AA axis). From this, it can be concluded that the crown height H1 of axis AA can be lower than the crown height H.sub.2 of axis PP without affecting the crown magnetic flux density.
This plate configuration although being optimum from the magnetic aspect, it is not economically satisfactory, since it inevitably produces material leftovers due to there being necessary scrap S between the strip edges and the plate contour (as shown in FIG.2) or between the plate contour (in this case the main coil slots are die-stamped towards the strip).
According to another known configuration of the plates, the material leftovers have been eliminated by die-stamping the plates with shaped-cuttings both towards the main coil slots (axis PP) and towards the auxiliary coil slots (axis AA) as illustrated in FIG. 3 for example. The axis PP could also be provided transversely to the longitudinal direction of the strip. In this configuration, the height H.sub.2 of the crown is reduced as a function of the shape-cuttings in the direction transverse to axis PP.
This second configuration as illustrated in FIG. 3, although being satisfactory from the economical aspect, is far from the magnetic aspect, since a reduction is caused in the magnetic flux passage, directly towards the main coil slots (axis PP), where the field induction has its maximum value. This magnetic flux reduction towards the main coil slots (axis PP) increases the crown magnetic losses substantially thereby impairing the motor efficiency.
Therefore there is an obvious need to maintain the height of the crown on axis PP, this being only possible should the slot height under the circular arc at the PP axis ends be reduced, the same way as it is done for AA axis intermediate coil slots. However, the slot height of the main coil is determined by the number of coils to be introduced in the slot, the allocation of which conventionally complies with a sine distribution, which is illustrated as an example in FIG. 4 for a 24 slot stator.
In FIG. 4, at the right side of the axis PP there is a group of 5 coil turns in the main coil, where the coil turns A (greater), B, C, D and E (smaller) may be seen. Each coil turn takes a fraction of the polar pitch, that to a 24 stator slots varies from 11/12 (greater coil turn) to 3/12 (smaller coil turn). As illustrated in FIG. 4, the coil percentage allocated to each coil turn (A, B, C and E) is obtained by the relationship: ##EQU1## per coil turn
In this known distribution the function f (.beta.) is given by EQU F (.beta.)=sin (90.beta.)
where .beta. is the fraction of stator polar pitch.
It is an essential condition for the motor that the harmonic content in any coil distribution used, either sine or decreasing order, will be as low as possible.
Should the harmonic content in any order be higher than the harmonics of the slots, a loss of torque occurs mainly in rotations from zero to 75% synchronous rotation. Harmonics having high amplitudes also increase iron magnetic losses, therefore reducing the motor efficiency.
In FIG. 4 there is shown Example 1 identifying a simple sine distribution corresponding to the sines of the angles which are identified by the fraction of the polar pitch. In Examples 2 to 5 the coil turns A and B have the same coil percentages, without any harmonic content increase. Thus, the large slots are sized by the volume of the coil turns A and B, the filling factor depending upon the manufacturing process being used.
Depending on the degree of any error in the number of coils caused by the manufacturing process, we can have coils at A in a larger number than at B or at B larger than at A; this can give a difference caused by the equipment with 2 to 4 coils. In the coil turns C, D and E, it is not necessary that the distribution follows the sine curve as shown in Examples 2 to 4 in FIG. 4. In each example analyzed, the harmonic content is very low, within the tolerable extent, without adversely affecting the iron losses. In the case of FIG. 4, it is also common to use 4 coil turns per group (see Example 5), therefore simplifying the manufacture process and the tools involved.
In FIG. 4, slots E and F are smaller due to the small volume of the auxiliary coil. Slots A and B are only used for the main coil and slot F is used only for the auxiliary coil. Therefore, it is noted that in the prior art examples shown, the usually adopted coil distribution does not allow that the height of slots A and B in the main coil be reduced. Such impossibility is due to the large number of coils to be introduced into such slots.